The present invention relates generally to blind detection of received signals having at least one property, which initially is unknown to the receiver. More particularly the invention relates to a method of identifying a property from a finite set of alternatives of an incoming signal according to the preamble of claim 1 and a blind signal detector according to the preamble of claim 21. The invention also relates to a computer program according to claim 19 and a computer readable medium according to claim 20.
Many communication standards provide examples of situations where a receiver must be able to receive a certain signal whose format, at least to some extent, is unknown. The receiver thus needs to perform a blind detection of the signal, i.e. no signaling takes place between the transmitter and the receiver before transmission according to the unknown format is initiated.
The GSM/EDGE-standard (GSM=Global System for Mobile communication; EDGE=Enhanced Data rates for Global Evolution) utilizes two different modulation schemes, namely GMSK (Gaussian Minimum Shift Keying) and 8PSK (Phase Shift Keying with eight different phase states). At start of transmission the transmitter may use any of these modulation schemes. Furthermore, during transmission, the modulation schemes can be changed, without notice, between every radio block (i.e. between every set of four consecutive bursts). This transmitter behavior, of course, requires a blind-detection capability of the receiver, at least with respect to said modulation schemes.
A theoretically conceivable solution would be to detect any received burst in parallel, both by means of a GMSK-equalizer and by means of an 8PSK-equalizer. This would result in two estimated sequences of bits that correspond to the received signal. A checksum/parity test could then be used to determine which modulation scheme that was actually applied when transmitting the sequence. The sequence detected under the incorrect modulation format would namely not pass such test. Naturally, the payload information contained in the received burst can also be derived through this solution simply by studying the sequence corresponding to the correct modulation format. However, the solution is far too computationally complex to be implemented in real time applications and is therefore not interesting from a technical point-of-view. There is yet no alternative solution either, which is satisfying in this respect.
The standard document ETSI Tdoc SMG2 EDGE 2E99-279, ETSI SMG2 Working Session on EDGE, Montigny Le Bretonneux, France, 24-27 Aug., 1999 presents a method for automatic detection of another unknown property of a received signal, namely a training sequence, and how to select an appropriate detection principle for the received signal. The document proposes that one out of three possible training sequences be identified according to the following procedure. First, a signal in the form of a radio burst is received. This signal is tested against a respective hypothesis for each of the three possible training sequences. The training sequence that corresponds to the hypothesis that results in the highest estimated signal power of the received burst is then selected as the training sequence having been used for the burst in question. Under ideal conditions, this procedure generally generates selection decisions of a sufficient accuracy. However, an actual radio environment is usually far from ideal. The received signal is hence more or less distorted by additive noise and/or interference signals. The interference signals typically originate from other radio stations, either transmitting at the same frequency/channel (so-called co-channel interference) or transmitting at an adjacent frequency/channel (so-called adjacent channel interference).
The demodulation schemes of today's radio communication systems normally include interference rejection algorithms in order to mitigate the effects of any undesired signal components in the received signal, such as noise and interference signals. In case a receiver in a system of this kind is required to make decisions pertaining to an unknown property of a received signal, and if these decisions per se do not involve interference rejection, there is a risk that the interference rejection algorithms with respect to the detected signal become useless, namely if, due to interference, an incorrect blind detection decision is taken. Hence, if a corresponding interference rejection is not also included in the blind detection procedure, this procedure is prone to be the limiting factor for the receiver performance, and consequently the entire system's performance. Presently, there exists no blind detection procedure, which involves interference rejection. Moreover, a direct inclusion of any of the known interference rejection algorithms into the known blind detection procedures would, again, impose a computation demand on the receiver, which is too high to be performed in real time.